A precision grinding and superfinishing process for a VVT solenoid valve spool

Abstract

The application discloses a precision grinding and superfinishing process for a VVT electromagnetic valve spool, and relates to the technical field of precision machining, aiming to solve the problems of precision loss of control and surface defects in grinding and superfinishing of the valve spool, and comprising the following steps: 1, workpiece pretreatment and tooling cleaning; 2, grinding wheel selection and pre-dressing; 3, precision cylindrical grinding parameter matching; 4, grinding cooling and temperature control optimization; 5, burr and surface detection after grinding; 6, superfinishing oil stone selection and parameter setting; 7, superfinishing auxiliary support and gap control; and 8, cleaning and final inspection after superfinishing. Through the workpiece pretreatment and tooling cleaning and the grinding wheel selection and pre-dressing procedures, the application realizes the elimination of tooling positioning impurity errors, the optimization of grinding wheel forming precision, the improvement of grinding size dispersion and the grinding wheel blockage and scrap problems, the reduction of single-piece repeated dressing downtime, the improvement of batch valve core size stability, and the solution to the precision defects caused by the original grinding tooling and grinding wheel.

Description

Technical Field

[0001] This invention relates to the field of precision machining technology, and more specifically, to a precision grinding and ultra-precision machining process for a VVT solenoid valve core. Background Technology

[0002] As a core control component of the engine's valve train system, the VVT ​​(Variable Valve Timing) solenoid valve's valve core machining precision directly determines the valve timing adjustment response speed, oil pressure control stability, and overall engine power performance and fuel economy. VVT solenoid valve cores are mostly made of high-strength alloy materials such as 38CrMoAl and GCr15, achieving a hardness of HRC58-65 after heat treatment. They have a slender shaft structure with multiple sets of oil control ring grooves, sealing edges, and stepped shaft sections. They must meet stringent geometric precision and surface quality requirements: cylindricity ≤1μm, coaxiality ≤4μm, mating clearance 3-8μm, and surface roughness Ra ≤0.05-0.2μm. Simultaneously, they must possess excellent wear resistance and sealing performance to prevent hydraulic oil leakage or valve core jamming.

[0003] Currently, the mainstream precision machining process for VVT solenoid valve cores is "precision external cylindrical grinding → ultra-precision machining." Precision external cylindrical grinding ensures dimensional accuracy and basic size formation, while ultra-precision machining removes grinding marks and achieves a mirror-like surface. Together, they guarantee the valve core's performance. However, due to limitations in valve core structure characteristics, material hardness, process parameter matching, and equipment tooling, existing processes have significant defects in actual operation, resulting in a high defect rate in valve core machining.

[0004] The specific defects are as follows:

[0005] 1. Defects in precision cylindrical grinding process

[0006] Precision cylindrical grinding, as a core process ensuring the dimensional and positional accuracy of valve cores, suffers from defects primarily manifested in three aspects: loss of geometric accuracy, deterioration of surface quality, and hidden damage.

[0007] In terms of geometric accuracy, the slender shaft structure of the valve core lacks rigidity. During grinding, the clamping force between the center and the elastic chuck can easily cause the workpiece to bend and deform. Combined with the spindle movement of the grinding machine and poor dynamic balance of the grinding wheel, this leads to shape errors in the valve core, such as waist-shaped, tapered, and rounded. After multiple clampings, the coaxiality of the multi-step structure exceeds the standard, directly causing uneven clearance between the valve core and the valve sleeve, resulting in local leakage or jamming under high pressure conditions. At the same time, the instantaneous temperature rise during the grinding process and the fluctuation of the ambient temperature cause thermal expansion and contraction of the workpiece. The deviation between the measured dimensions in the cold state and the processed dimensions in the hot state becomes a hidden cause of batch dimensional instability.

[0008] In terms of surface quality, excessive grinding feed and insufficient cooling spray can easily lead to surface tempering burns and micro-cracks in the workpiece, especially in 38CrMoAl nitrided parts. The debris generated by the cracks can contaminate the hydraulic oil. Improper selection of grinding wheel grit, clogging or passivation, or chipping of edges and corners can leave spiral grinding marks and scratches of varying depths on the valve core surface. Micro-burrs of 0.003 to 0.008 mm are easily formed at the root of the annular groove. These are difficult to completely remove in subsequent ultra-precision machining, directly affecting the sealing performance and valve core life.

[0009] In terms of tooling and grinding wheel operation, if the grinding debris stuck to the end face of the center and the flexible chuck is not cleaned in time, it will introduce positioning errors, causing workpiece end face runout and random deviations in radial dimensions; fine-grained grinding wheels are prone to clogging and self-sharpening imbalance, requiring frequent maintenance and machine stoppages, which seriously affects production efficiency.

[0010] 2. Defects in ultra-precision machining processes

[0011] The core objective of ultra-precision machining is to remove grinding marks and reduce surface roughness to Ra≤0.05μm. However, existing processes are prone to causing microscopic defects and deterioration of geometric accuracy during operation.

[0012] Regarding surface micro-defects, improper matching between the oilstone oscillation frequency and the workpiece rotation speed can cause disordered ultra-precision texture and excessive local roughness. Residual grinding marks from previous grinding processes can reduce the sealing performance of the sealing surface. Excessive ultra-precision pressure and insufficient cleanliness of lubricating and cooling oil can cause detached fine abrasive particles to be pressed into the valve core surface, forming an "ultra-precision nodule." Subsequent engine oil flushing and shedding of these particles as impurities become the core hidden danger of VVT valve jamming. During ultra-precision, the sealing edge is prone to micro-collapse and arc deformation due to excessive oilstone pressure, losing its chip-breaking ability and exacerbating the risk of valve jamming.

[0013] Regarding the deterioration of geometric accuracy, the lack of central auxiliary support during ultra-precision of slender valve cores causes radial pressure from the honing stone to lead to a concave depression in the middle of the workpiece, resulting in a concave / convex bulge-shaped error. For valve cores with multi-ring groove structures, uneven contact pressure across the grooves during ultra-precision can cause inconsistent ultra-precision allowances on both sides of the ring grooves, leading to unilateral exceedance of radial clearance and significant differences in leakage between high and low pressure conditions. Furthermore, ultra-precision honing stones wear quickly, exhibit poor batch consistency, and lack control over the amount of trace removal, easily resulting in under-polishing or over-polishing. Manual parameter adjustment is insufficient to achieve fully closed-loop control.

[0014] Based on the above problems, we propose a precision grinding and ultra-precision machining process for the valve core of VVT solenoid valves. Summary of the Invention

[0015] The purpose of this invention is to overcome the shortcomings of the prior art, adapt to practical needs, and provide a precision grinding and ultra-precision machining process for the valve core of a VVT solenoid valve, so as to solve the problems of uncontrolled precision and surface defects in valve core grinding and ultra-precision machining.

[0016] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a precision grinding and ultra-precision machining process for a VVT solenoid valve core, comprising the following steps:

[0017] Step 1: Pre-treatment of workpiece and cleaning of tooling. The heat-treated valve core blank is ultrasonically cleaned to remove surface oxide scale and oil stains. At the same time, a high-pressure air gun is used with alcohol to wipe the top, end face of the elastic chuck and positioning surface to remove residual grinding debris and debris, ensuring that the flatness of the positioning surface is ≤0.001mm.

[0018] Step 2: Grinding wheel selection and pre-dressing. Select diamond grinding wheels with a grit size of W40 to W10. Use a diamond dressing pen for online pre-dressing. The dressing speed is 1 / 3 of the working speed of the grinding wheel. The dressing feed rate is 0.002 to 0.005 mm / r. After dressing, the dynamic balance accuracy of the grinding wheel is ≤2 g・mm.

[0019] Step 3: Matching of precision external cylindrical grinding parameters. Three-stage grinding is adopted: rough grinding, semi-finish grinding and finish grinding. The feed rate for rough grinding is 0.01~0.02mm / r, semi-finish grinding is 0.005~0.01mm / r, and finish grinding is 0.001~0.003mm / r. During grinding, the valve core is supported by a double center support. The center clamping force is controlled at 0.3~0.5MPa. The spindle speed of the grinding machine is matched with the linear speed of the grinding wheel at 1500~2000r / min and 30~45m / s, respectively.

[0020] Step 4: Grinding cooling and temperature control optimization. A dual-path high-pressure cooling system is adopted, with the coolant temperature controlled at 20±2℃, the nozzle distance from the grinding zone ≤15mm, and the cooling pressure 0.8~1.2MPa. At the same time, the processing environment temperature is stabilized at 20±1℃ to avoid thermal deformation.

[0021] Step 5: Post-grinding burr and surface inspection. After fine grinding, the root of the ring groove is treated with electrolytic deburring process to remove micro burrs of 0.003 to 0.008 mm. Then, the surface roughness is detected by white light interferometer and the cylindricity and coaxiality are detected by laser diameter gauge.

[0022] Step 6: Selection and parameter setting of superfinishing oilstone. Select a cubic boron nitride oilstone with a Shore A hardness of 85-90, set the oilstone oscillation frequency to 80-120 times / min, the workpiece rotation speed to 300-500 r / min, the contact pressure between the oilstone and the workpiece to 0.1-0.3 MPa, and the superfinishing time to 3-5 min.

[0023] Step 7: Ultra-precision auxiliary support and clearance control. During ultra-precision, an adjustable flexible center support is used, with the support point located at 1 / 2 of the valve core length. For multi-ring groove valve cores, the pressure sensor provides real-time feedback on the contact pressure of the oilstone across the groove, and the oilstone feed is dynamically adjusted to ensure that the ultra-precision allowance deviation on both sides of the ring groove is ≤0.001mm.

[0024] Step 8: Cleaning and final inspection after ultra-precision. After ultra-precision, a three-stage filtration cleaning process is used to remove residual grinding debris and ultra-precision nodules from the surface. Then, the surface roughness is detected by atomic force microscopy and the amount of sealing edge collapse is detected by optical microscopy. After passing the inspection, the product is put into storage.

[0025] Preferably, in step 3, the support block of the double center frame is made of polytetrafluoroethylene. The surface roughness Ra of the support surface of the support block is Ra≤0.02μm after polishing. The contact area between the support surface and the valve core is controlled at 5~8mm². The curvature of the arc-shaped contact surface of the support block is consistent with the curvature of the outer circle of the valve core, and the fit is ≥95%.

[0026] Preferably, in step 4, the coolant is an emulsion containing extreme pressure additives, and the extreme pressure additives are a compound system of sulfurized olefins and phosphate esters; at the same time, a coolant circulation filtration system is set up, which adopts a two-stage filtration structure, with the first stage filtration being a 10μm metal filter screen and the second stage filtration being a 5μm glass fiber filter element.

[0027] Preferably, in step 6, the swing trajectory of the oilstone is a sine curve, the swing amplitude is set to 5-8 mm, and the phase of the swing trajectory maintains a fixed proportional relationship with the workpiece rotation speed, with a proportionality coefficient of 1:1.5-1:2.5; the working layer thickness of the oilstone is 8-12 mm, the internal structure adopts a honeycomb pore structure with a porosity of 40-50%, and the end face of the oilstone is chamfered with a chamfer angle of 15-20°.

[0028] Preferably, in step 7, the flexible center support is hydraulically driven, and the support force is infinitely adjustable via a proportional relief valve, with an adjustment range of 0.05–0.2 MPa and a control accuracy of ±0.005 MPa. The support head of the flexible center support is made of hard alloy material with a titanium nitride coating on the surface. The support system is linked with the honing stone pressure control system. When the pressure sensor detects a change in the honing stone cross-groove contact pressure exceeding 0.02 MPa, the hydraulic system adjusts the support force in real time to maintain the ratio of support force to honing stone pressure between 1:2 and 1:3, ensuring that the valve core remains in a stable force balance state during the ultra-precision process.

[0029] Preferably, after the grinding wheel is pre-dressed in step 2, 3 to 5 valve cores need to be tested and verified. The same grinding parameters as in formal production are used during the test grinding. After the test grinding is completed, the outer circle contour accuracy of the valve core is detected by an optical projector to ensure that the contour tolerance is ≤0.002mm. At the same time, the grinding texture on the surface of the valve core is observed by a scanning electron microscope.

[0030] Preferably, in step 5, the electrolytic deburring is performed using a pulse electrolysis method, with the electrolysis voltage set to 12-15V, the pulse frequency to 500-1000Hz, the duty cycle to 30-50%, and the electrolysis time controlled to 10-20s.

[0031] Preferably, the final magnetic particle inspection in step 8 adopts a wet continuous magnetization method. The magnetic suspension is a mixture of fluorescent magnetic powder and kerosene, with a magnetic powder concentration of 10-20 g / L, a magnetization current of 500-800 A, and a magnetization time of 2-3 s. During the inspection, the valve core surface is irradiated with an ultraviolet lamp with a wavelength of 365 nm and a light intensity ≥1000 μW / cm².

[0032] Preferably, in step 3, the grinding wheel linear velocity of the three-stage grinding process (rough grinding, semi-finish grinding, and finish grinding) gradually increases. The grinding wheel linear velocity is 30-35 m / s in the rough grinding stage, 35-40 m / s in the semi-finish grinding stage, and 40-45 m / s in the finish grinding stage. This gradient increase in linear velocity is combined with a gradual decrease in feed rate. At the same time, a micro-feed mode is adopted in the finish grinding stage, with a 0.5-1 s pause after each feed, allowing sufficient time for heat to dissipate in the grinding zone and reducing thermal deformation caused by instantaneous temperature rise.

[0033] Preferably, in step 6, real-time wear monitoring of the oilstone is used during the ultra-precision process. The feed compensation of the oilstone is detected by a displacement sensor. When the compensation reaches 0.05 mm, it is determined that the oilstone has entered the late stage of wear. At this time, the contact pressure of the oilstone is automatically adjusted, and the pressure value is increased by 10-15% on the original basis. At the same time, the ultra-precision time is extended by 0.5-1 min to compensate for the decrease in removal capacity caused by oilstone wear. When the compensation exceeds 0.1 mm, the machine is automatically stopped and the oilstone is prompted to be replaced.

[0034] Compared with the prior art, the beneficial effects of the present invention are:

[0035] 1. This invention eliminates tooling positioning errors and optimizes grinding wheel forming accuracy through workpiece pretreatment, tooling cleaning, and grinding wheel selection and pre-dressing processes. It also improves the problems of grinding dimension dispersion and grinding wheel clogging and scrapping. Before processing, the tooling positioning surface is thoroughly cleaned with alcohol and high-pressure air to strictly control the flatness of the positioning surface and prevent positioning deviations caused by debris. By limiting the grinding wheel specifications and standardizing dressing parameters, and combining them with trial grinding verification processes, the consistency of the grinding wheel profile is ensured. This effectively reduces grinding defects such as edge roundness, taper, and spiral scratches, reduces the frequency of grinding wheel clogging, reduces downtime for repeated dressing of single parts, and improves the dimensional stability of batch valve cores, thus solving the accuracy defects caused by the original grinding tooling and grinding wheel.

[0036] 2. This invention achieves the suppression of bending deformation during grinding of slender valve cores and the reduction of grinding thermal deformation and surface burn crack defects through a three-stage gradient grinding process, dual-center support, and dual-path constant temperature cooling and temperature control. It adopts a graded feed of roughing, semi-finishing, and finishing grinding with segmented incremental grinding wheel linear speed, and a dual-center support with large-fit buffer support to reduce workpiece bending under pressure. Dual-path fixed-point high-pressure cooling combined with constant temperature control in the workshop, and the coolant compounded with extreme pressure additives quickly remove the high temperature of grinding, resulting in an instantaneous temperature drop in the grinding zone. This effectively solves the two major industry pain points of deformation and thermal damage during grinding of slender shafts, and improves the pass rate of valve core cylindricity and coaxiality.

[0037] 3. This invention achieves thorough removal of micro-burrs at the root of the annular groove through pulse electrolysis deburring and graded inspection processes, preventing burrs from scratching the sealing surface and causing potential VVT valve jamming due to detachment. The invention uses pulse electrolysis to precisely remove burrs from the groove root, relying on pulse parameter control to avoid excessive corrosion of non-machined surfaces. Post-electrolysis cleaning removes residual electrolyte. After fine grinding, a white light interferometer and laser diameter gauge are used for comprehensive surface roughness and dimensional accuracy inspection, pre-screening defective semi-finished products and preventing defective workpieces from entering the ultra-precision process. This eliminates the potential for stubborn burrs, achieves pre-process quality control, reduces assembly failures caused by burrs, and improves the reliability of the valve core seal.

[0038] 4. This invention improves defects such as disordered texture, workpiece bulging, ultra-precision edge collapse, and over- or under-polishing in ultra-precision machining by optimizing honing stone parameters, flexible hydraulic linkage support, graded cleaning, and comprehensive final inspection. This invention limits the CBN honing stone material, sinusoidal oscillation trajectory, and speed ratio to form a uniform cross-texture. The hydraulic flexible support is linked with the honing stone pressure, and real-time pressure adjustment across the groove eliminates uneven allowance on both sides of the annular groove. Online monitoring of honing stone wear automatically compensates for pressure and time, avoiding under- or over-polishing. Three-stage ultrafiltration cleaning after ultra-precision machining removes embedded particles, and final inspection using fluorescent magnetic particle testing combined with morphology detection checks for cracks and edge damage, thus reducing bulging error, ultra-precision edge collapse, and other defects. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the process flow of the present invention;

[0040] Figure 2 This table compares the performance parameters of the VVT ​​valve core processed by the process of this invention with those processed by traditional methods. Detailed Implementation

[0041] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0042] A precision grinding and ultra-precision machining process for a VVT solenoid valve core is disclosed. The valve core material is 38CrMoAl, and the hardness after heat treatment and nitriding is HRC60~63. The valve core has an outer diameter of φ5mm and a total length of 48mm. It has three oil control ring grooves and sealing edges on both sides. The target cylindricity is ≤1μm, coaxiality is ≤4μm, and Ra≤0.05μm. The process includes eight steps: Step 1: Pre-treatment of workpiece and cleaning of tooling. After heat treatment, the valve core blank is put into an ultrasonic cleaning tank and ultrasonically cleaned at room temperature for 12 minutes to remove surface oxide scale and heat treatment oil stains. Before machining, the center and elastic chuck are disassembled. The positioning end face is wiped with a soft cloth soaked in anhydrous alcohol and fine grinding debris is blown away with 0.6MPa high-pressure dry air. The positioning surface is checked with a flat crystal to ensure that the flatness of the positioning surface is ≤0.001mm and there are no particulate impurities left before the workpiece is clamped.

[0043] Step 2: Grinding wheel selection and pre-dressing. Select a W20 grit diamond grinding wheel and perform online dressing with a diamond dressing pen. The dressing speed is 1 / 3 of the rated working speed of the grinding wheel, and the dressing feed is 0.003 mm / r. After dressing, the dynamic balance of the grinding wheel is corrected to ≤2 g・mm. According to the requirements of claim 6, four samples are continuously test-ground. The outer diameter tolerance is ≤0.002 mm as measured by an optical projector. The grinding pattern is uniform and free of breaks and chipping as observed by SEM. After passing the test, mass production is started.

[0044] Step 3: Precision cylindrical grinding parameter matching is divided into three levels: rough grinding, semi-finish grinding, and finish grinding. Rough grinding feed is 0.015 mm / r, grinding wheel linear speed is 32 m / s; semi-finish grinding feed is 0.007 mm / r, linear speed is 38 m / s; finish grinding feed is 0.002 mm / r, linear speed is 43 m / s; finish grinding pauses for 0.8 s for heat dissipation after each feed. The double center support block uses Shore D58 PTFE, arc-shaped to fit the outer circle of the valve core, with a fit of 96% and a contact area of ​​6 mm²; the tailstock center air supply pressure is 0.4 MPa, the grinding machine spindle speed is 1800 r / min, and the double support system suppresses bending of the slender shaft during grinding.

[0045] Step 4: Grinding Cooling and Temperature Control Optimization. The machining workshop is kept at a constant temperature of 20±1℃. Dual cooling nozzles are arranged on both sides of the grinding wheel, with the nozzles 12mm away from the grinding zone. The coolant pressure is 1.0MPa. The coolant is a compound extreme pressure emulsion of sulfurized olefins and phosphate esters, with a kinematic viscosity of 24mm² / s at 40℃. The circulation pipeline is equipped with a 10μm coarse filter and a 5μm glass fiber fine filter. Every 8 hours, a particle counter is used to detect oil impurities, controlling ≥5μm particles to ≤50 particles / mL, continuously inhibiting the circulation of grinding debris that scratches the workpiece.

[0046] Step 5: Deburring and surface inspection after grinding. After fine grinding, pulse electrolysis is used to remove burrs: voltage 13V, pulse frequency 750Hz, duty cycle 40%, electrolysis time 15s; 0.003~0.008mm burrs in the ring groove are completely removed; then ultrasonic cleaning with deionized water at 45℃ for 6min, followed by drying with hot air at 70℃; white light interferometer is used to check the roughness of the outer circle, and laser diameter gauge is used to check the cylindricity and coaxiality. Unqualified parts are isolated and reworked.

[0047] Step 6: Selection and parameter setting of the ultra-precision oilstone. The selected oilstone is a Shore A87 cubic boron nitride oilstone with a working layer thickness of 10mm, a honeycomb porosity of 45%, and an 18° chamfer on the end face. The oilstone has a sinusoidal oscillation amplitude of 6mm, an oscillation-to-workpiece speed ratio of 1:2, an oscillation frequency of 100 times / min, a workpiece speed of 400r / min, an oilstone static pressure of 0.2MPa, and an ultra-precision time of 4min for a single piece. The displacement sensor monitors the wear of the oilstone in real time. When the feed compensation reaches 0.055mm, the pressure is increased by 12%, the ultra-precision time is delayed by 0.7min, and the machine automatically stops and the oilstone is replaced when the wear compensation exceeds 0.1mm.

[0048] Step 7: Ultra-precision auxiliary support and clearance control. The flexible hydraulic support head is arranged at the midpoint of the valve core axis. The support head has a hard alloy substrate and a titanium nitride coating. The support pressure is 0.12MPa, and the pressure adjustment accuracy is ±0.005MPa. During the machining of the multi-ring groove, the pressure sensor collects the oilstone pressure in real time. If the pressure fluctuation is >0.02MPa, the hydraulic system is immediately linked to adjust the support force. The support force and oilstone pressure are kept at 1:2.5. The ultra-precision allowance difference between the left and right sides of the ring groove is ≤0.001mm.

[0049] Step 8: Ultrafiltration cleaning and final inspection: Three-stage cleaning in sequence: 20μm coarse filtration spray → 5μm fine filtration immersion → 3μm ultrafiltration ultrasonic cleaning, to thoroughly remove free abrasive particles and embedded ultrafine nodules on the surface; Final inspection: Atomic force microscopy to measure surface Ra, optical microscopy to check for collapsed sealing edges; Wet fluorescent magnetic particle inspection: magnetic particle concentration 15g / L, magnetization current 650A, 365nm ultraviolet lamp inspection, to detect surface microcracks, and after passing all items, it is put into storage.

[0050] Example 1

[0051] Processing object: VVT valve core made of 38CrMoAl material, φ5mm×48mm, with 3 sets of oil control ring grooves, mass production of 1000 pieces, processed using the complete set of processes described above in this invention. Processing results: Valve core cylindricity 0.5~0.8μm, coaxiality 2.2~3.5μm, outer diameter Ra 0.03~0.045μm; no grinding burns or surface cracks, burrs in the ring grooves are completely removed; ultra-precision machining with no concave / convex deformation, no ultra-precision edge, and sealing edge collapse <0.002mm; overall defect rate 0.8%.

[0052] Comparative Example

[0053] 1000 valve cores of the same specification and material were produced using conventional grinding followed by ultra-precision grinding with a regular oilstone. There was no constant temperature cooling, no central flexible support, no electrolytic deburring, and no online monitoring with the oilstone. The results showed: cylindricity 1.2~2.5μm, coaxiality 4.5~7μm, and Ra>0.2μm for some workpieces. Grinding burns, groove root burrs, ultra-precision bulging, and ultra-precision nodules were frequent, resulting in an overall defect rate of 8.7%. After installation, the VVT ​​jamming and internal leakage failure rates were significantly higher than expected.

[0054] The embodiments disclosed in this invention are preferred embodiments, but are not limited thereto. Those skilled in the art can easily understand the spirit of this invention based on the above embodiments and make different extensions and variations, but as long as they do not depart from the spirit of this invention, they are all within the protection scope of this invention.

Claims

1. A precision grinding and ultra-precision machining process for the valve core of a VVT solenoid valve, characterized in that, Includes the following steps: Step 1: Pre-treatment of workpiece and cleaning of tooling. The heat-treated valve core blank is ultrasonically cleaned to remove surface oxide scale and oil stains. At the same time, a high-pressure air gun is used with alcohol to wipe the top, end face of the elastic chuck and positioning surface to remove residual grinding debris and debris, ensuring that the flatness of the positioning surface is ≤0.001mm. Step 2: Grinding wheel selection and pre-dressing. Select diamond grinding wheels with a grit size of W40 to W10. Use a diamond dressing pen for online pre-dressing. The dressing speed is 1 / 3 of the working speed of the grinding wheel. The dressing feed rate is 0.002 to 0.005 mm / r. After dressing, the dynamic balance accuracy of the grinding wheel is ≤2 g・mm. Step 3: Matching of precision external cylindrical grinding parameters. Three-stage grinding is adopted: rough grinding, semi-finish grinding and finish grinding. The feed rate for rough grinding is 0.01~0.02mm / r, semi-finish grinding is 0.005~0.01mm / r, and finish grinding is 0.001~0.003mm / r. During grinding, the valve core is supported by a double center support. The center clamping force is controlled at 0.3~0.5MPa. The spindle speed of the grinding machine is matched with the linear speed of the grinding wheel at 1500~2000r / min and 30~45m / s, respectively. Step 4: Grinding cooling and temperature control optimization. A dual-path high-pressure cooling system is adopted, with the coolant temperature controlled at 20±2℃, the nozzle distance from the grinding zone ≤15mm, and the cooling pressure 0.8~1.2MPa. At the same time, the processing environment temperature is stabilized at 20±1℃ to avoid thermal deformation. Step 5: Post-grinding burr and surface inspection. After fine grinding, the root of the ring groove is treated with electrolytic deburring process to remove micro burrs of 0.003 to 0.008 mm. Then, the surface roughness is detected by white light interferometer and the cylindricity and coaxiality are detected by laser diameter gauge. Step 6: Selection and parameter setting of superfinishing oilstone. Select a cubic boron nitride oilstone with a Shore A hardness of 85-90, set the oilstone oscillation frequency to 80-120 times / min, the workpiece rotation speed to 300-500 r / min, the contact pressure between the oilstone and the workpiece to 0.1-0.3 MPa, and the superfinishing time to 3-5 min. Step 7: Ultra-precision auxiliary support and clearance control. During ultra-precision, an adjustable flexible center support is used, with the support point located at 1 / 2 of the valve core length. For multi-ring groove valve cores, the pressure sensor provides real-time feedback on the contact pressure of the oilstone across the groove, and the oilstone feed is dynamically adjusted to ensure that the ultra-precision allowance deviation on both sides of the ring groove is ≤0.001mm. Step 8: Cleaning and final inspection after ultra-precision. After ultra-precision, a three-stage filtration cleaning process is used to remove residual grinding debris and ultra-precision nodules from the surface. Then, the surface roughness is detected by atomic force microscopy and the amount of sealing edge collapse is detected by optical microscopy. After passing the inspection, the product is put into storage.

2. The precision grinding and ultra-precision machining process for the VVT ​​solenoid valve core according to claim 1, characterized in that, In step 3, the support block of the double center frame is made of polytetrafluoroethylene. The surface roughness Ra of the support block is Ra≤0.02μm after polishing. The contact area between the support surface and the valve core is controlled at 5~8mm². The curvature of the arc-shaped contact surface of the support block is consistent with the curvature of the outer circle of the valve core, and the fit is ≥95%.

3. The precision grinding and ultra-precision machining process for the VVT ​​solenoid valve core according to claim 2, characterized in that, In step 4, the coolant is an emulsion containing extreme pressure additives, which are a compound system of sulfurized olefins and phosphate esters. At the same time, a coolant circulation filtration system is set up, which adopts a two-stage filtration structure. The first stage filtration is a 10μm metal filter screen, and the second stage filtration is a 5μm glass fiber filter element.

4. The precision grinding and ultra-precision machining process for a VVT solenoid valve core according to claim 3, characterized in that, In step 6, the swing trajectory of the oilstone is a sine curve, the swing amplitude is set to 5-8mm, and the phase of the swing trajectory maintains a fixed proportional relationship with the workpiece rotation speed, with a proportionality coefficient of 1:1.5-1:2.5; the working layer thickness of the oilstone is 8-12mm, the internal structure adopts a honeycomb pore structure with a porosity of 40-50%, and the end face of the oilstone is chamfered with a chamfer angle of 15-20°.

5. The precision grinding and ultra-precision machining process for a VVT solenoid valve core according to claim 4, characterized in that, In step 7, the flexible center support is hydraulically driven, and the support force is infinitely adjustable via a proportional relief valve, with an adjustment range of 0.05–0.2 MPa and a control accuracy of ±0.005 MPa. The support head of the flexible center support is made of hard alloy material with a titanium nitride coating on the surface. The support system is linked with the honing stone pressure control system. When the pressure sensor detects a change in the honing stone cross-groove contact pressure exceeding 0.02 MPa, the hydraulic system adjusts the support force in real time to maintain the ratio of support force to honing stone pressure between 1:2 and 1:3, ensuring that the valve core remains in a stable force balance state during the ultra-precision process.

6. The precision grinding and ultra-precision machining process for a VVT solenoid valve core according to claim 5, characterized in that, After the grinding wheel is pre-dressed in step 2, 3 to 5 valve cores need to be tested and verified. The same grinding parameters as in formal production are used during the test grinding. After the test grinding is completed, the outer circle contour accuracy of the valve core is detected by an optical projector to ensure that the contour tolerance is ≤0.002mm. At the same time, the grinding texture on the surface of the valve core is observed by a scanning electron microscope.

7. The precision grinding and ultra-precision machining process for a VVT solenoid valve core according to claim 5, characterized in that, In step 5, the electrolytic deburring adopts a pulse electrolysis method, with the electrolysis voltage set to 12-15V, the pulse frequency to 500-1000Hz, the duty cycle to 30-50%, and the electrolysis time controlled to 10-20s.

8. The precision grinding and ultra-precision machining process for a VVT solenoid valve core according to claim 5, characterized in that, In step 8, the final magnetic particle inspection adopts a wet continuous magnetization method. The magnetic suspension is a mixture of fluorescent magnetic powder and kerosene, with a magnetic powder concentration of 10-20 g / L, a magnetization current of 500-800 A, and a magnetization time of 2-3 s. During the inspection, the valve core surface is irradiated with an ultraviolet lamp with a wavelength of 365 nm and a light intensity ≥1000 μW / cm².

9. The precision grinding and ultra-precision machining process for a VVT solenoid valve core according to claim 5, characterized in that, In step 3, the grinding wheel linear velocity gradually increases in the three stages of grinding: rough grinding, semi-finish grinding, and finish grinding. The grinding wheel linear velocity is 30-35 m / s in the rough grinding stage, 35-40 m / s in the semi-finish grinding stage, and 40-45 m / s in the finish grinding stage. This gradient increase in linear velocity is combined with a gradual decrease in feed rate. At the same time, a micro-feed mode is adopted in the finish grinding stage, with a 0.5-1s pause after each feed to allow sufficient time for heat to dissipate in the grinding zone, reducing thermal deformation caused by instantaneous temperature rise.

10. The precision grinding and ultra-precision machining process for a VVT solenoid valve core according to claim 5, characterized in that, In step 6, real-time wear monitoring of the oilstone is used during the ultra-precision process. The feed compensation of the oilstone is detected by a displacement sensor. When the compensation reaches 0.05 mm, it is determined that the oilstone has entered the late stage of wear. At this time, the contact pressure of the oilstone is automatically adjusted, and the pressure value is increased by 10-15% on the original basis. At the same time, the ultra-precision time is extended by 0.5-1 min to compensate for the decrease in removal capacity caused by oilstone wear. When the compensation exceeds 0.1 mm, the machine automatically stops and prompts for oilstone replacement.

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